In large scale, prior art photovoltaic power plants, a great number of solar photovoltaic modules are connected in series and then in parallel to facilitate DC power collection at a central location where it can then be converted to AC power by a DC-to-AC power converter or inverter.
Typically, the sizing of conductors in any electrical installation is based on how much current a conductor can handle and remain at a safe temperature. In photovoltaic power plants, the value of the photovoltaic energy is high and conductors are oversized with respect to ampacity in order to limit the overall resistive wiring losses of the system. In a properly designed photovoltaic power plant, the incremental cost to increase the size of a conductor to save one watt of resistive losses should be equal to the cost of installing an additional watt of photovoltaic generating capacity. As such, the cost of collecting power from acres of solar panels is a significant portion of the overall power plant cost. If a megawatt power plant can be built for $5/Watt AC, then a 1% reduction in copper conductor losses could save $50,000 in system costs.
If the maximum power point voltage of each solar module could be individually tracked, the overall energy harvest from the photovoltaic power plant would be maximized. This extreme approach would not be cost effective. However, as the number of maximum power point trackers in a system is increased, the annual energy harvest will be increased as well. In large power plants, sections of the total array will have different wind exposures, different local soil reflectivity, different cloud cover, different soiling, different shadowing, different aging characteristics and different “factory” module characteristics. All of these factors will affect the maximum power point voltage of any group of modules. If one large maximum-power-point-tracking DC-to-AC inverter converts the entire array power, this power converter will operate at an average PV operating point. The portions of the array that are statistically weaker or stronger will not operate at their maximum power point voltages and array harvest will be compromised. A number of tradeoffs need to be considered, however, for any system design between complexity (and therefore reliability), power conversion inefficiencies of the maximum power point trackers, system costs and array harvest enhancements.
FIG. 2 illustrates a typical, prior art photovoltaic power plant. Photovoltaic sub-array 20 is a collection of series and parallel connected photovoltaic modules. Conductors 27 and 28 carry the combined current of sub-array 20 in conduit 29 to DC-to-AC converter inputs 5 and 6. This circuit path from a large subarray to the DC-to-AC converter is commonly referred to as a home run. Conductors 27 and 28 are indicated in FIG. 2 as resistors to represent the total resistance of the conductors for this home run. In a similar way, photovoltaic subarray 30 is connected to DC-to-AC converter inputs 7 and 8 via conductors 37 and 38 in conduit 39. FIG. 2 only shows two home runs, for clarity, but the number is variable depending on the system design and photovoltaic array layout. In large power plants, the distance traversed by a given home run can be substantial. To achieve efficient DC power collection in any power plant, it is desirable to make the operating voltage of subarrays 20 and 30 as high as practical. Higher voltage translates to lower current for a given power level and therefore smaller conductor cross sectional area resulting in lower conductor and conduit costs. Typically, the maximum voltage is limited by the photovoltaic module voltage rating from active elements to frame or external insulating surfaces.
In FIG. 2, DC-to-AC converter inputs 5 and 7 are connected to fuses 9 and 10 respectively then electrically paralleled to one side of capacitor 4. DC-to-AC converter inputs 6 and 8 are electrically paralleled to the remaining side of capacitor 4. The current and voltage characteristic of subarray 20 or 30 is that of an imperfect voltage source or an imperfect current source, depending on the operating point of the subarray. As such, the power source “seen” at the DC-to-AC converter inputs is “soft” with limited voltage and limited current. Capacitor 4 serves to convert this soft source to a low impedance voltage source capable of delivering high peak currents which are orders of magnitude greater than what either subarray could deliver. DC-to-AC converter output terminals 1, 2 and 3 are connected to a polyphase electric utility grid. The utility grid is modeled as AC voltage sources 11, 12 and 13. For each phase, the DC-to-AC converter regulates sinusoidal current into the utility grid in phase with the utility voltage at each output terminal 1, 2 and 3 to source power into the grid at unity power factor. The sinusoidal current sources within the DC-to-AC converter 1T/1B, 2T/2B and 3T/3B are modeled as controllable current sources capable of sourcing regulated half-sinewaves of current into the positive half-sinewave of utility voltage and sinking regulated half-sinewaves of current out of the negative half-sinewave of utility voltage for each phase.
To summarize, the most salient points of this discussion and how they relate to the invention, FIG. 2 illustrates DC sources 20 and 30 converted to a combined DC voltage source by capacitor 4 in turn converted to a polyphase AC current source to transfer power into a polyphase AC voltage source, the electric utility grid.